1. Field of the Invention
The present invention relates to the field of water vehicles, and particularly water vehicles of the hydrofoil type.
2. Prior Art
Water vehicles of a wide variety of configurations are well known in the prior art. Of particular importance to the present invention are water vehicles of relatively small size designed to operate on the surface of the water, and to be supported thereby during the higher speed operation thereof, not by the displacement of a substantially equal mass of water, but instead by the hydrodynamic forces on the hull (typically referred to as planing) and/or through the use of hydrofoils piercing and operating below the surface of the water. Planing type hulls of course are well known, though in recent years various new configurations for such hulls have been produced in substantial quantities. Typically such hulls are characterized by a v-shaped, curved forward section for slicing into the water as required, with the v becoming shallower and shallower, sometimes significantly approaching a flat bottom towards the stern for sliding over the surface of the water. Therefore, the forward section of the hull is designed to gradually engage the water, whereas the center and particularly the rearward portion of the hull is designed to be supported by the water through relatively low hydrodynamic pressures distributed over the area. Thus, planning may be achieved at a reasonably low speed. However, such hulls are characterized by a very large wetted area and therefore relatively high and steadily increasing viscous drag occurs with higher speed operation. The newer configurations for such hulls include hulls that approach twin and triple hull configurations, both of which operate however in a similar manner.
Hydrofoils, on the other hand, may be characterized as water piercing supporting members in which the support is derived not only from the increased hydrodynamic pressure on the bottom surface of the hydrofoil, but also from the decreased dynamic pressure of the water flowing over the top surface of the hydrofoil. Thus the hydrofoil is much like the wing of an airplane, and effectively "flies" at a controlled position beneath the surface of the water.
The similarities between hydrofoils as they operate in water and airfoils as used for wings etc., of airplanes are substantial. While air is a compressible medium, negligible compression occurs while flying at speeds well below the speed of sound so that the air essentially behaves as it it were incompressible. As in an aircraft, dynamic stability of a water vehicle is a most important consideration, for the vehicle must operate not only in "static" equilibrium but must survive and return to the static condition when perturbed by wave motion, shifts in weight of the vehicle, etc. and underwater disturbances. In this regard, stability about the yaw axis and about the roll axis of a water vehicle may be achieved in much the same manner as it is achieved for an airplane, that is, stability about the yaw axis by a significant rudder type surface behind the center of gravity of the vehicle, and about the roll axis, either as a result of the dihedral of the hydrofoils etc., or specifically in the water vehicle situation by the reduced lift of a hydrofoil as it approaches or pierces the surface of the water. However, for a water vehicle using hydrofoils, the problems about the pitch axis are particularly severe for a number of reasons. Some of these problems include the fact that the hydrofoils, by necessity, are not located vertically near the center of gravity of the vehicle, since a very large percentage of the total mass of the vehicle is supported by hydrofoils above the water, whereas the hydrofoils must extend below the surface of the water by a proper amount. Thus, changes in drag on the hydrofoils due to changes in proximity of the hydrofoil to the water surface, turbulence in the water, etc., result in a substantial pitching moment on the vehicle. In this regard it should be noted that, particularly with smaller vehicles in large bodies of water having substantial wave motion, ground swells, etc., it is quite necessary that the depth of penetration of the hyrofoils be reasonably controlled so that the vehicle may generally follow the wave contour, as opposed to ploughing into the face of the wave. (In larger vehicles control is also required, though generally not with respect to wave motion since such vehicles generally are supported above the surface of the water by the hyrofoils by an amount equal to or greater than normal wave action).
Another factor or problem encountered, which is unique to the hydrofoil situation in comparision to airfoils on airplanes, is that there is a very sharp discontinuity in the density and other characteristics of the medium near the desired operating position of the hydrofoil, e.g. the air-water interface at the surface of the water. Thus, the depth of penetration of the hydrofoil must be carefully controlled if it is to operate as a hydrofoil and not attempt to rise to the surface of the water to merely operate as a small planing surface. Consequently, in the prior art, even with large hydrofoil vehicles, a control system has been used to essentially control the angle of attack of the hydrofoils to provide the required stability, and to control the depth of penetration of the hydrofoils for most optimum operation thereof.
Hydrofoil control systems for larger vehicles, while being relatively expensive, provide the desired result and are economically justifiable because of the improved speed, etc., of the vehicle over a more conventional hull type. However, since a certain amount of mechanism is required to provide hydrofoil control responsive to the proper operating parameters, and often for small vehicles even greater control is required since smaller vehicles must generally follow smaller, more frequent disturbances, the cost of the control system is not decreased in proportion to other decreases in cost for smaller vehicle. Consequently, while many hydrofoil configurations have been proposed for use in small recreational vehicles, there is at the present time no commercially successful small hydrofoil vehicle for recreational purposes adapted to carry a single individual or a small number of passengers. Thus, while there has been a proliferation of various types of off-the-road land vehicles for recreational purposes, such as trail bikes, dune buggies, etc., there has not been a proliferation of recreational vehicles of a similar character for water use, primarily because of the lack of efficient and practical design for high performance, small water vehicles. This has been true even though there are presently in this country hundreds of thousands of outboard motors suitable for propelling such vehicles, whereas each land vehicle typically requires as a part thereof a special limited purpose and permanently installed engine. Thus, there is a need for water vehicles of the recreational type to duplicate, on water, the speed, stability, performance and excitement achieved with motorcycles and dune buggies on land, which are capable of highly responsive yet safe performance, and which preferably utilize conventional outboard engines of a reasonable range of power so as to minimize the cost of such vehicles for persons already owning or having access to such an outboard engine.